The dispersed fringe sensor has been applied as a piston error detector for James Webb Space Telescope (JWST) and Giant Magellan Telescope (GMT) in United State. However, since it is influenced by noise of a dispersed fringe image and the method for extracting the piston error, the dispersed fringe sensor is only utilized during a rough co-phasing stage for these two telescopes.
At present, a method for improving a detecting capability of a dispersed fringe sensor in a fine co-phasing stage is proposed (cf. W. Zhao and G. Cao, “Active co-phasing and aligning testbed with segmented mirrors”, Opt. Express, 19(9), 8670-8683 (2011)). In particular, the method extracts an interference image in one dimension of the interference direction from a dispersed fringe image, and then extracts a displacement when a piston error of an offset of main peak in an interference intensity distribution of the one dimensional image is zero, and finally calculates a piston error required for the fine co-phasing in conjunction with a fringe cycle of a corresponding wavelength of the one-dimensional interference image:piston=Δy/T(λ).
Although such a method enhances a detecting precision of the dispersed fringe sensor and meets the requirements of the fine co-phasing, such a method has a poor capability of resisting image noise since it only utilizes one dimensional information of the two dimensional dispersed fringe image. During actual operation of an astronomical telescope, an illumination of start light functioning as a light source is weak and there is noise in a target surface of a camera, so a signal-to-noise ratio of the dispersed fringe image at this moment will not reach the same ideal level as that of the laboratory. Thus, an actual value in use for such a method is not good enough. Furthermore, such a method needs to calibrate the wavelength corresponding to different spatial positions of the target surface of the camera and calibrate the position at which the piston error of the main peak in the interference intensity distribution is zero, which increases engineering complexity and cost without doubt. More importantly, the present method is not sensitive to the translation movement of the pixel of the image, and has strong engineering feasibility which the current fine method (MPP) does not have.